Ethylene-vinyl alcohol copolymers (hereinafter, may be abbreviated as “EVOH(s)”) are useful polymeric materials that are superior in barrier properties against various types of gases such as oxygen, oil resistance, antistatic properties, mechanical strength and the like; therefore, EVOHs have been molded into films, sheets, containers, packaging materials and the like, and widely used as various types of packaging materials. However, the EVOH has a large number of hydroxyl groups in its molecule, and has high crystallinity, a great velocity of crystallization and poor flexibility; therefore, the EVOH has disadvantages that: the EVOH exhibits poor suitability for secondary processing, in particular, heat stretching properties when being molded into packaging materials for foods and the like, leading to the generation of cracks in the molding, and in turn, to a decrease of a process yield of products; and a reduction of mechanical strength due to a thickness variation and/or deterioration of gas barrier properties occur(s), leading to lack of quality stability.
In order to improve the suitability for secondary processing, blending various types of elastomers such as an ethylene-vinyl acetate copolymer (EVA) into the EVOH has been proposed. However, these elastomers have a disadvantage that they have poor compatibility with the EVOH and therefore the resultant composition may have reduced transparency.
In view of such drawbacks, as a method for improving the transparency while ensuring the suitability for secondary processing, a process including blending EVOHs each having a different ethylene content has been developed. Specifically investigated examples include the following: a resin composition that contains two types of saponified ethylene-vinyl acetate copolymers, one having a degree of saponification of 95 mol % or more and the other having a degree of saponification of 70 mol % or more, and a polyamide resin in which a terminal carboxyl group is adjusted (see Japanese Unexamined Patent Application, Publication No. H8-239528); a resin composition that contains EVOH having a degree of saponification of 98 mol % or more, and EVOH having a reduced degree of saponification through reacetylation (see Japanese Unexamined Patent Application, Publication No. 2000-212369); and a resin composition that contains three types of EVOHs each having a different ethylene content (see Japanese Unexamined Patent Application, Publication No. 2001-31821).
However, according to the aforementioned conventional technologies, the transparency and the heat stretching properties are improved, but due to the use of the polyamide resin or the reacetylated EVOH, they have disadvantages that: a property for operation for a long time period (long-run workability) is poor; the generation of gels and seeds (dirt under paint) is increased in a long-run operation; and the like. In addition, in such a method involving using different EVOHs, flow marks, which are not typically generated in a single-EVOH system, are generated, and in particular significantly generated in a long-run operation. The flow marks are generated when the difference of viscosity is caused between two types of EVOHs in an operation for a long time period, leading to an unstable flow that occurs at a front part of the flow (“flow front” as generally referred to), in the melt molding. These flow marks as well as coloring are causes for unfavorable appearance in the resultant products.